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Studies on the coating of fuel particles for the ‘dragon’ reactor experiment
Journal of Nuclear Energy. 1967. Vol. 21. pp. 623 m 642.
Pergamott Press Ltd.
Printed in Northern Ireland
STUDIES ON THE COATING OF FUEL PARTICLES FOR THE ‘DRAGON’ REACTOR EXPERIMENT J. R. C. GOUGH* and D. KERNf (First received 4 January 1967 and injinalform
1 March 1967)
Abstract-The requirements of fission product retaining coatings on coated fuel particles are reviewed and the scope of development necessary to meet these requirements for the initial charges of the DRAGON Reactor Experiment is described. These studies on the coating of fuel particles were carried out using a laboratory scale fluidized bed type of coating furnace. Within the range of deposition temperatures investigated, laminar or metallographically structureless pyrolytic carbon was deposited with methane concentrations greater than 15 per cent, while the structure deposited at low methane concentrations was columnar. Appreciable temperature gradients were found in these small fluid&d beds of fuel particles, particularly under the relatively static conditions of fluidization which favoured deposition of the columnar structure. The rate of deposition of pyrolytic carbon was found to increase linearly with methane concentration at a given temperature, and within specified limits of methane concentration such that the relationship was linear within each of two separate ranges of concentration corresponding to the different metallographic structures. The room temperature density of the deposited pyrocarbons showed minimum values for deposition temperatures of 1600-1700°C. Under the experimental conditions described, the carbon efficiency of deposition increased to the order of 100 per cent at deposition temperatures of 1600°C and above. The deposition of silicon carbide interlayers from methyltrichlorosilane is described. The surface contamination of coated particle fuel was shown to be sensitive to the temperature of deposition of the pyrocarbon, to the rate of deposition at a fIxed temperature, and to the presence of silicon carbide interlayers. The latter reduced surface contamination to near background values. 1. INTRODUCTION
THE development of high-temperature gas-cooled reactors depends to a large extent on the integrity of the fission product retaining coatings on coated fuel particles. The fuel particle coatings are required to fulfil the following functions: (a) To facilitate fuel processing and assembly, not only by reducing contamination levels but also by preventing the hydrolysis which would otherwise occur in the case of thorium containing carbide fuel. (b) To protect the fuel from reaction with impurities in the coolant or with matrix materials. (c) To prevent or reduce to a minimum, migration or decomposition of the fuel. (d) To delay or retain gaseous and solid fission products to high burn-up levels in order to facilitate maintenance of the primary circuit, to reduce the load on or to eliminate entirely the fission product clean-up plant, and to reduce shielding
requirements.
The primary development of fuel particles coated with pyrolytic carbon and silicon carbide for the DRAGONReactor Experiment has been described in a paper presented at the Battelle Symposium on Ceramic Matrix Fuels Containing Coated Particles (1962). Fuel element fabrication for the reactor experiment was described in a paper (PRICE et al., 1966) presented at the British Nuclear Energy Society Symposium on High-Temperature Reactors and the DRAGONProject. The purpose * DRAGONProject, APE, Winfrith, Dorchester, Dorset. t Deceased, previously DRAGON Project. 623 1
624
J. R. C. Goucm and D. KERN
of the present paper is to give the results of studies on the coating of fuel particles carried out in the period up to the fabrication of the initial fuel charges for DRAGON. 2. REQUIREMENTS
OF THE
COATING
The many requirements to be met by the coatings on fuel particles range from those affecting the fabrication processes to those related to irradiation performance. It is desirable for example that any vapour deposited coating can be applied at a temperature above that of reactor operation so that differential expansion will not ‘&use fracture or prestressing of the coating. Similarly the thermal expansion of the coating should be not greater than that of the fuel kernel, otherwise the coating will tend to fracture on cooling from the coating temperature, as in the case of MgO deposited on to dense UO,. However, differential expansion effects may be alleviated by porous, spongy interlayers. The coating should be free from deleterious impurities, for example, adsorbed or occluded hydrogen, which might increase the neutron absorption cross section or react chemically with the fuel or the coating. However, most gaseous and volatile impurities are readily removed by vacuum heat treatment which is normally carried out after consolidation of the fuel, and has the additional objective of stabilising the dimensions of the fuel compacts. Additionally, the coating should be relatively free from fuel contamination originating from the kernel, in order that the release of fission products shall be minimized. Such contamination can arise from thermal shock and/or self-abrasion of fuel kernels in the early stages of the coating procedure, followed by incorporation of the fuel dust in the deposited pyrocarbon. A further possible cause of the formation of fuel dust is chemical reaction between the fuel kernels and impurities either in glove box controlled atmospheres, or in the carrier and coating gases in the fluidized bed or tumbling furnace used to deposit the coatings. Contamination of coatings can also be caused by fuel diffusion from the kernel during the actual coating operation. The diffusion of uranium, for example, through pyrocarbon as deposited in the early stages of development became significant at coating temperatures of 1550°C and above, while the radium-228 and radium-224 daughter products of thorium diffuse rapidly at even lower temperatures. Dispersion of the coated particle fuel in a graphite matrix requires that the coating should have sufficient mechanical strength to withstand the processes of mixing and subsequent pressing or extrusion. The proportion of coatings whose breakage can be tolerated will depend in any particular application on the fission product retention properties of the kernel itself and of the matrix, whether fission product release is controlled by the use of a purge flow, and so on. However, the incidence of fracture of coatings can be minimized by appropriate modification or special development of techniques, including for example, ‘overcoating’ with matrix material prior to consolidation. The physical and chemical properties of the coating must satisfy many requirements. For example, the coating should be compatible with the fuel, matrix and coolant under operational conditions. It should have a high melting point or decomposition temperature, low volatility, show no phase changes up to maximum operating temperatures, and must maintain its integrity under fast neutron irradiation and thermal cycling. The neutron absorption cross section should be acceptably low.
FIG. I.-Zirconium-uranium
monocarbide fuel for the first charge of the Reactor Experiment. [x 1251
FIG. 2.-Thorium-uranium
dicarbide fuel for the first charge of the Experiment. [X 1251
DRAGON
DRAGON
Reactor
624
FIG. 3.-Laboratory
scale fluidizing furnace.
FIG. 5.-Duplex
FIG. 6.-Sintered
pyrolytic carbon coated fuel particle showing laminar and columnar structures. [ X 2001
thorium-uranium dicarbide coated with two layers of laminar pyrolytic carbon at 1600 and 1900°C. [x 1251
FIG. 7.-Enriched melted thorium-uranium dicarbide particles with ‘interrupted laminar’ pyrolytic carbon coating deposited at 1500°C. [ x 1251
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
625
In order to achieve good retention properties for gaseous and volatile fission products the coating is required to be of low permeability. The diffusion coefficients for fuel and solid and gaseous fission products should be low under service conditions, and in this respect the small lattice parameter of silicon carbide is particularly attractive. The selection of the optimum density of coating layers is dominated by the fact that high density and an isotropic structure are required to minimize fast neutron irradiation induced shrinkage (BOKROSand PRICE,1965) while porous inner layers are desirable to accommodate fission product gases and the swelling of fuel kernels. These requirements and the need to delay the diffusion of fuel and fission products have led to the development of duplex and triplex coatings giving the optimum combination of properties for fission product retention. Finally it is essential that the coating material should be economic not only from the point of view of initial fabrication, but also in that it should not adversely affect the economical reprocessing of the fuel. 3. SCOPE
OF DEVELOPMENT
The first charge of fuel elements for the DRAGON Reactor Experiment was arranged as a two-zone core, the inner zone containing fuel in the form of thorium-uranium dicarbide while in the outer annulus the fuel was zirconium-uranium monocarbide. In both cases the coated fuel particles were dispersed in a matrix consisting substantially of graphite. The total fissile loading of 14 kg of 2W at an enrichment of 93 per cent by weight was distributed uniformly throughout the core. The thorium ratio in the inner zone was Th: 235U= 10: 1 by atoms, while the zirconium ratio in the outer zone was Zr: 235U= 8 : 1 by atoms. The zirconium-uranium monocarbide fuel particles were of nominal diameter 251-422 p and were coated with two layers of pyrolytic carbon, the inner being deposited at 1600°C and the outer at 1450°C (Fig. 1). In the case of the thoriumuranium dicarbide fuel particles the kernel diameter was nominally 353-500 ,u and the triplex coating consisted of an inner layer of pyrolytic carbon deposited at 15OO”C, an intermediate layer of silicon carbide also deposited at 1500°C and an outer layer of pyrolytic carbon deposited at 1450°C (Fig. 2). In the second charge of the Reactor Experiment the zirconium-uranium monocarbide fuel has been replaced by fuel consisting of a dispersion of uranium dicarbide in excess carbon. The latter kernels are of diameter 420-572 ,u and are coated with pyrolytic carbon and a 35 ,u interlayer of silicon carbide to give fuel particles of 710-1000 ,LJ overall diameter. The pyrolytic carbon coatings were deposited at higher temperatures than in the case of the first charge, and were therefore of higher density and more isotropic. A number of advanced thorium-uranium carbide and oxide fuels were also developed and are being irradiated in the second charge. Plutonium-bearing coated particle fuel is also being evaluated by irradiation in the second charge of the Reactor Experiment. The early development of coating technology within the DRAGON Project had as its immediate objective the coating of small batches (<40 g) of fuel particles with pyrolytic carbon for irradiation experiments. The first fuel particles to become available were porous, sintered uranium dicarbide of diameter 144-251 p and the
626
J. R. C. GOUGHand D. KERN
coating deposited was plain pyrolytic carbon. Subsequent development resulted in the production of fuel kernels of zirconium-uranium monocarbide and thoriumuranium dicarbide with zirconium : uranium and thorium : uranium ratios of up to 8 : 1 and 15 : 1 by atoms respectively. These had a kernel diameter of 251-500 p and were coated either as sintered (mercury density 3.5-6.5 g cm-3) or as melted (mercury density 6-8 g crnA) with pyrolytic carbon, and in some cases with an interlayer of silicon carbide. More recent developments have included the preparation of 353572 ,u kernels of uranium dicarbide with a large excess of carbon, a tendency to increase kernel diameters to satisfy heavy metal concentration requirements, the development of plutonium bearing fuels, and a progressive increase in the temperature of deposition of pyrocarbon coatings. The selection of optimum temperatures of deposition and concentrations of hydrocarbon has been shown (BOKFIOSand PRICE) to give pyrocarbons having favourable values of structural parameters (density, isotropy and crystallite size) to minimize fast neutron irradiation induced shrinkage. 0
Argon Feeding
purge
Feeding container
tub
Fluidizing
odiation
FIG. 4.-Schematic
vessel
shields
diagram of laboratory-scale fluidtig
furnace.
The purpose of the DRAGON fuel irradiation programme has been to confirm that the fuel selected for the tist and subsequent reactor charges will withstand the service conditions of burn-up and temperature. These objectives have involved the development of appropriate coating procedures and detailed investigations into the rBle of the various coating parameters. All the information presented in this paper has been obtained using the laboratory scale fluidized bed type of coating furnace shown in Figs. 3 and 4. Variations in the concentration of hydrocarbon were known to give rise to a distinctive change of metallographic structure from laminar to columnar as shown in Fig. 5. One of the first objectives of the fuel irradiation programme was to compare the fission product retention properties of these two structures alone or in combination, deposited at various temperatures in the range 1450-1900°C. Another objective was
Studies on the coating of fuel particles for the DRAGON reactor experiment
627
to investigate whether discontinuities in the deposited coating in the form of a change of structure, a gap, or an interlayer of for example silicon carbide, improved the irradiation performance of the fuel by acting as effective barriers to the diffusion of fuel and fission products. Under certain conditions of coating, pyrolytic carbon tends to be deposited within the pores of sintered kernels rather than as a layer around the kernel. As this could significantly affect the amount of free volume available within the kernel for the accommodation of gaseous and volatile fission products, it was necessary to determine the extent of this effect. In the case of pyrolytic carbon the coating variables of principal interest are the temperature of deposition, which affects the density of the coating and the rate of deposition; thehydrocarbonconcentration, which affects the metallographic structure, the rate of deposition and to some extent density; and the time of coating. The dependence of carbon efficiency, rate of deposition and density of the pyrocarbon coating on these variables and on the weight and density of the charge have been studied in detail. However, carbon efficiency and rate of deposition in laboratory-scale fluidized beds are markedly dependent on temperature distributions. The latter were therefore investigated as a function of gas flow, weight of charge and the density of the fuel particles. 4. DEPOSITION
OF PYROLYTIC
CARBON OF VARIOUS STRUCTURES
4.1 Coating agents Pyrolytic carbon has been deposited on fuel particles in the DRAGONProject laboratories at optically measured temperatures between 1400°C and 1900°C (and recently at temperatures well in excess of 2OOO”C),by vapour decomposition of hydrocarbon gases. The large majority of experiments were carried out using methane as coating agent, but propane, butane and acetylene were investigated as alternative sources of pyrolytic carbons particularly when porous deposits were required. Carbon tetrachloride vapour has been used as a gas leaching agent to reduce the surface contamination of coated particle fuel by the formation and removal of volatile halides of uranium and thorium; during this leaching process pyrolytic carbon was formed as an outer sealing layer at a very low rate of deposition. Table 1 gives typical compositions of the argon carrier gas and of the methane, the latter being used as coating agent in the large majority of experiments. The impurity contents of carrier gases and coating agents are recognized as possible causes of reaction with kernel materials and hence pollution of the vapour deposited coating TABLE l.-TYPICAL COMposll-IONSOF ARGGN CARRIERGAS AM) OF METHANEUSED IN THE COATINGOF FUEL PARTICLES
Hydrogen Argon
(99.995 “/,I Mkthane -. (85.6 %>
41 v-pm
-
Nitrogen
Carbon dioxide
Carbonaceous matter determined as carbon dioxide
15vpm
-
lvpm
lvpm
150 vpm
10.32%
0.94 %
-
Oxygen
Water vapour
2vpm 3.14%
628
J. R. C. Gouorr and D. KERN
with radioactive debris. To minimize the influence of moisture, coating and carrier gases were dried by passage through a column of molecular sieves and by this means moisture levels of 4-6 vpm were obtained. 4.2 The laminar structure of pyrolytic carbon In accordance with the results of other investigations in this field (Proceedings of BOKROS,1964) the metallographic appearance of pyrocarbons deposited onto fuel particles in the temperature range 1400-1900°C was laminar or structureless when the methane concentration was 15-50 per cent of the total gas flow. The distinctively laminar metallographic structure is characterized by the presence of circumferential rings or laminations which may be caused by fluctuations of temperature or methane concentration experienced by individual fuel particles in the fluidized bed. The effect is probably enhanced by relatively slow circulation of the fuel particles, particularly in the case of deep beds. With the coating conditions used in the present investigation, however, the pyrolytic carbons deposited have normally been found to show no apparent metallographic structure (Fig. 6). the Symposium on Ceramic Matrix Fuels Containing Coated Particles;
4.3 The columnar structure of pyrolytic carbon It is well known that the pyrolytic carbon obtained by deposition on to free surfaces of for example heated rods, or furnace walls, has a characteristic cone structure (CARLEY-MACAULEY and MACKENZIE,1963). A similar structure is shown when pyrolytic carbon coatings are deposited on to fuel particles using tumbling furnace techniques. In the course of the development of the technology of coating fuel particles within the DRAGONProject, investigations have been carried out to define the conditions under which pyrolytic carbon coatings of columnar structure are obtained using laboratory-scale fluidized bed furnaces. Deposition of this structure has been found to be favoured by relatively static conditions of fluidization, for example at low gas flows, in fluidizing reactors of wide cone angle, and when fuel kernels of high density and/or large diameter are fluidized. 4.4 Pyrolytic carbon with intermediate discontinuities In the DRAGONfuel irradiation programme, the inner layer of pyrolytic carbon deposited on to fuel particles has sometimes contained deliberately introduced circumferential discontinuities which are intended to act as barriers to the diffusion of fuel and fission products and to prevent the propagation of cracks through the coating. Several techniques investigated to obtain these discontinuities are described below. On the basis of previous experience of the deposition of pyrolytic carbon of various structures, a technique has been developed which consists of decreasing the hydrocarbon concentration from that used for the laminar structure to that for the columnar structure for an interval sufficient to give a zone 2-3 p thick, followed by reversion to the conditions for a laminar structure (Fig. 7). Several techniques found to be unsuccessful included cooling partially coated particles within the furnace followed by reheating to the coating temperature; interruption of the flow of methane for a short period; increasing the methane
Studies on the coating of fuel particles for the DRAGON
reactor experiment
629
concentration to a high value for a short period; and cooling the particles down to 77°K after removal from the fluidizing bed in order to achieve an interruption by thermal contraction of the kernel. Previous experience had shown that circumferential discontinuities sometimes form in the coating close to the kernel during cooling from coating temperatures to ambient, but that this only occurs in the case of sintered kernels of relatively high density; the effect can therefore vary from fuel particle to fuel particle in the same coating batch. Removal of partially coated fuel into the laboratory atmosphere followed by continuation of the coating process was partially successful but did not give reproducible results. 5. TEMPERATURE
DISTRIBUTIONS SCALE FLUIDIZED
IN LABORATORY BEDS
Although the movement of fuel particles inside fluidized bed furnaces has been studied by carrying out model experiments at room temperature using higher gas flows to simulate the conditions at normal deposition temperature, it has been difficult to draw valid conclusions from the results obtained because of the major influence of temperature in fluidization techniques. Also, in order to obtain detailed information on the kinetics of the deposition of pyrolytic carbon on fuel particles, an essential preliminary was the measurement of temperature distributions inside the fluidixed bed reactor tube. A systematic investigation was carried out on the effects of density of fuel particles, weight of charge and flow rate of carrier gas on temperature distributions in laboratory-scale coating furnaces of this type. A uniform distribution is desirable from the point of view of consistency of the product; furthermore the thermal cycling inherent in some coating processes may have an influence on fission product retention properties.
5.1 Experimentalresults In the temperature range investigated the total gas flow had only slight influence on the temperature distribution, other conditions being constant (Fig. 8). The depth of the bed of fuel particles and the uniformity of temperature distribution increased with weight of charge (Fig. 9). The density of the fuel particles had a marked influence on the pattern of temperature distribution, the average particle temperature decreasing with increase in particle density. However, this effect was less pronounced in the case of 40 g charges, (Figs. 10 and 11). The maximum temperature gradients in the bed of fuel particles were found under conditions normally used for the deposition of columnar pyrolytic carbon coatings, that is at low gas flows in fluidizing reactors of wide cone angle. These gradients measured by traversing a thermocouple through the fluidized bed of fuel particles were compared with the temperature measured optically at a fixed position on the heater tube as shown in Fig. 4. Under conditions simulating the deposition of columnar pyrolytic carbon the average fuel particle temperature was 310°C below that measured optically. A 50 per cent increase in gas flow decreased this difference by only 45°C (Fig. 12). In view of the experimental observations on temperature gradients in the laboratoryscale fluidized bed it was desirable to obtain a more uniform particle temperature over the whole range of the fluidized bed depth. By this means the temperature peak which is pronounced for 20 g charges and also evident for 40 g charges would be obviated. The requirements to preheat the carrier gas and to avoid excessive loss
J. R. C. &NJOH and D. KJJRN
630
160IO -
I
140 10 -
Y 120 d 3 0 ; z c”
.
IOC IO
8C IO
Temperature: Maferial: 2Og X No particles 0 200 MO,C . 2Og M&C v
200
1460*C prccoalcd Mo$ (p=b59g/cm3) flow 1515 cm3/min flow ISIS cdlmin flow 2035 cm3hnin
MO&
flow
2555
I
cn?/min
I 6C 10
I
0
Distance
FIG. I.-Temperature
800-
30
20
IO
from
nozzle,
cm
distribution of the fluidized bed fitted with a normal graphite nozzle (28’) at various flows of carrier gas.
Tempwofurc: l460.C Gas flow: I515 cdfmln X No porticlcs 0 200 Mozt (p=2.59g/cm3) v 409 Mo,C (p=2.59g/cm31
Distance
from
nozzle,
cm
FIG. 9.-Influence of charge on the temperature distribution of the fluid&d bed.
Studies on the coating of fuel particles for the DRAGONreactor experiment
160
140
Y
120
90
Temperature: 1460.C Gas flow: 1515 cm%nin Charge: 209 + No particles x MO& p=259q/cm3(12~5cm3) 0 tZr, UtC 7.44: lip=5.37g/cm3 v
uCz
(3M)
60
,p=t0.45g/cm3
FIG. lo.-Infkence
cm’1
! 20
IO Distance
t6~9cdt (3.0
nozzle,
from
‘;
‘30
i-
cm
of particle density on the temperature bed (20 g charge).
distribution
of the fluidized
+ 5 a
1600
1400
‘x \
\
y7
‘+ \
’\
’\
“\\
___
“I
8OC
Temperature: l460.C Gas flow: 1515 cd/min Chorqe: 400 + No parlicles X Mo*C. p=2,59g/cm3 0 (Zr, WC, 7.44: Ii p=5.37g/cd v ucz
600
(3M)
p=IO~45~/cm3
IO Distance
FIG. 11.-Influence
20 from
nozzle,
cm
of particle density on the temperature bed (40 g charge).
distribution
of the fluidked
631
J. R. C. OouoEI and D.
632
Distance FIG.
from nozzle,
cm
12.-Temperature distribution of the fluidized bed using conditions suitable for depositing columnat pyrolytic carbon layers.
Distance FIG.
KERN
13.-Inlluence
from lube inlcl,
cm
of charge on the temperature distribution of the fluidized bed using the improved nozzle.
Studies on the coating of fuel particles for the
I
I
DRAGON
I J.
I
I
reactor experiment
5 IO I5 20 25 Distance from nozzle inlet, cm
Distance
FIG. 14.-Comparison
Y
1700
m- 1600 Z! 0
from
nozzle
inlet,
cm
of the two nozzle systems to determine the influence of particle density on the temperature distribution.
Temperature: 1460. -- -
409 charge Improved no~zle:209charge 406 charge
z g I500 'ij 2 1400 3 ? $I300 E
1%
2 % 1200
-_
’ 1.
._
!! u a
-_ -.
IlOO
5
4
5 Particle
6
7 density,
6 g/cm’
9
IO
II
FIG.15.--The influence of particle density on the average particle temperature.
633
634
J. R. C. GGUGH
and D. Ih3RN
of heat to the water-cooled gas injection assembly led to a graphite nozzle design whose principal features were an extended oril%e between the gas injection assembly and the conical base of the fluidizing reactor and the incorporation of a replaceable inner sleeve. The new design gave a signilicantly more uniform temperature distribution of the fuel particles; however, a slight temperature peak was still observed in the case of 20 g batches (Fig. 13). The temperature of the experiment within the range investigated had little or no influence on the pattern of temperature distribution. Temperature distributions were much less affected by changes in the density of fuel particles in the case of the improved nozzle than they were for the previous nozzle (Fig. 14). This point is further illustrated in Fig. 15. 6. INFLUENCE OF THE COATING VARIABLES ON THE DEPOSITION OF PYROLYTIC CARBON
6.1 Coating thickness The sintered particles produced by the DRAGON powder agglomeration process have sometimes been characterized by sufficient open porosity to allow carbon to be deposited within the kernel during the initial stages of the coating process. A series of experiments were carried out to make a quantitative estimate of the amount of carbon deposited into typical porous kernels at 1600°C. The results obtained (Fig. 16) indicate that during the initial 5 min of coating, 12 wt. % or 30 vol. % carbon is deposited into these kernels. The relationship between coating thickness and weight increase at 1500°C is similar to that at 1600°C except that the coatings corresponding to a given weight increase are thinner. This observation is in agreement with density measurements, which show minimum values when the pyrocarbons are deposited between 1600°C and 1700°C. 6.2 Rate of deposition The rate of deposition of pyrolytic carbon on to fuel particles increases considerably with increase in fuel particle density (the weight of charge being constant), as the same weight of deposited carbon is distributed amongst a smaller number of particles. The dependence of the rate of deposition on the concentration of methane at a coating temperature of 1600°C is linear as shown in Fig. 17. The relationship between the rate of deposition and the reciprocal of the absolute temperature of coating is also linear (Fig. 18). In the case of 4.2 per cent methane concentration and 15 g charges, a 90” included angle graphite nozzle and a total gas flow of only 670 cm3 min-1 (instead of 1520 cm3 min-l) were used in order to obtain the relatively static conditions required to achieve columnar deposition. 6.3 Coating density The room temperature density of vapour deposited pyrolytic carbon of both the laminar and columnar metallographic structures varies with the temperature of deposition and shows maximum values -2-O g cm-s for temperatures of 1000°C and below, and 1800°C and above. Minimum density values of about l-75 g cm-3 at 1600°C were reported in the case of the columnar structure and l-5 g cm-3 at 1450°C in the case of the laminar structure (Proceedings of the Symposium on Ceramic
Studies on the coating of fuel particles for the 60-
I
Totol flow 1470cm’Imin Charge 13.5 g methone
50
I t $a
concentrotionz50%
40
IO
20
Coating
FIG. 16.-Variation
j_
30
time,
min
of weight increase and coating thickness with time of coating 1600°C for (Zr, U)C/8 : 1 sintered particles.
Material: Charge:
e
/
/
0
.iii 0
Zirconium-uranium 13.59
monocarbide
40-
n
0
IO
I 30
20
Methone FIG.
17.-Dependence
Material: -
4.8
1900
FIG. 18.-Variation
.
Thorium-uronium 29.3% Methone 10.9% Methane 4.2% Methone
I
I
5-o t
t
1900
- - -
I
I
I
46
40
50
%
concentration.
of deposition rate at 1600°C on methane concentration.
-
0
635
reactor experiment
DRAGON
5.2 t 1700
5.4 t I600
dicarbide
concentration concentration concentration
20g 209 l5g
charge charge charge
I
1
56
t I500
of deposition rate with temperature
5.8 t 1450
I/OK x IO4
OC
(l/K).
at
636
J. R. C. GOUGH and D.
I "'800
Fro. lg.-Variation
I 1000
I 1200 Deposition
I 1400 temperature,
KERN
I 1600 T
I I800
2‘0130
of density of pyrolytic carbon coatings on fuel particles with temperature of deposition.
Matrix Fuels Containing Coated Particles). These results are plotted in Fig. 19 in conjunction with density values measured in the course of the present investigations. Density values were determined by crushing the coated particles, leaching away the kernel material and measuring the density of coating fragments by the ‘sink and float’ method using liquids of standard density. It can be seen from Fig. 19 that under the conditions of the present laboratory scale coating experiments, the temperatures of deposition giving the minimum values of coating density are in the range of MO1700°C and that the methane concentration and therefore rate of deposition appears to have no pronounced i&rence on coating density. However there is a tendency for the coatings deposited at a low rate of deposition to show slightly higher density values, especially at the maximum temperature of deposition, which was 1900°C in this series of experiments. DIEFE~RF (1960) has shown that the characteristic minimum in the curve of coating density against temperature of deposition becomes less pronounced with decrease of pressure and finally disappears when the coating is applied under very much reduced pressure, e.g. 1.7 x low2torr.
6.4 Carbon ejiciency The dependence of the carbon efficiency of deposition (defined as the ratio of carbon deposited on the fuel particles to that contained in the coating agent) on the time of coating and on the methane concentration is illustrated in Figs. 20 and 21 respectively. The influence of the temperature of deposition on the carbon efficiency for the laminar, fine-grained transitional, and columnar metallographic structures is shown in Fig. 22, from which it is apparent that for any given temperature of deposition, higher carbon efficiencies are obtained with decreasing concentrations of
Studies on the coating of fuel particlti for the
100
.o
t
Material:
Zirconium-uranium
DRAGON
monocarbide
35.8%
u 0
IO
Fro. 20.-Variation
1 20
reactor experiment
I
I
I
30 Time.
melhone
50
40 min
I
60
‘r
of carbon efkiency with time of coating for various methane concentrations at 16WC.
Materiot: Zirconium-uranium 13.59 charge
monocarbide
J
zo-
I IO
0 FIG.
21.-Influence
I
20 Methane
I
40
I
SO’
%
of metbane concentration on carbon efficiency for laminar and columnar coating structures deposited at 1600°C.
Temperature, FIG.
I
30 concentration,
22.4nfluence
‘c
of temperature on carbon efficiency.
637
638
J. R. C. GOUOH and D. KERN
methane. Although relatively low carbon efficiencies (of the order of 30-40 per cent) have been observed at coating temperatures of 1450°C and below, the values for each metallographic structure of pyrolytic carbon increased to the order of 100 per cent at 1600°C and above. 7.
THE
DEPOSITION
OF
SILICON
CARBIDE
AT
1500°C
Although relatively volatile carbide-forming fission products such as strontium, barium and caesium are not retained by pyrocarbon coatings to the same degree as the rare gases (FLOWERS et al., 1962), it has been shown that their release is considerably reduced by the incorporation of interlayers of silicon carbide (DE NORDWALL, 1967; private communication), Techniques for the application of silicon carbide coatings were therefore developed by the DRAGON Project and fuel with silicon carbide as an intermediate coating layer was tested in various irradiation experiments (SAYERS et al., 1963). Silicon carbide deposited at 1470°C from methyltrichlorosilane vapour in a hydrogen-argon mixture was dense and of uniform thickness with a smooth outer surface. Increasing the temperature of deposition to 1670°C resulted in apparent porosity and a distinctly rougher outer surface although the inner showed little if any change. For these reasons a temperature of 1500°C was chosen as the deposition temperature for silicon carbide coatings in irradiation specimens. Using a hydrogen carrier concentration of 50 per cent smooth layers could be obtained, which were not interlocked with the adjacent pyrocarbon layers (Fig. 23). With decreasing concentration of hydrogen in the feed gas mixtures, progressive deterioration in the quality of the SIC layer occurred as for example in Fig. 24 showing coatings obtained using only 235 per cent hydrogen. In these experiments only the proportion of hydrogen in the silane carrier gas flow was varied, the total flow of argon and hydrogen through the silane being kept constant. With particles in a density range of 3.54~5 g cm-s deposition rates of 30-50 p/hr were obtained using 40-50 g charges. 8. SURFACE
CONTAMINATION
Release of gaseous fission products to the extent of 10-l or 10V2of their rate of generation occurs through coatings which have been fractured, or completely penetrated by the diffusion of fuel. In the case of coatings of high integrity, release values of the order of 10-s or 1O-6 have been observed and have been correlated with fuel contamination in the coating. Surface contamination is defined as being within the range of cc-particle emission, i.e. ~15-20 ,u in pyrolytic carbon. The main causes of surface contamination are the formation of fuel dust by abrasion, thermal shock or reaction with impurities in the carrier and coating gases, with the subsequent incorporation of the dust within the coating, and the diffusion of fuel through the coating. It is therefore essential to adopt all possible measures to reduce these effects by for example maintaining the purity of the feed gases, ensuring that the fuel kernels are of adequate quality as regards crushing strength and abrasion resistance, charging the kernels into the coating furnace at a temperature such as to avoid excessive thermal shock, and applying the initial coating layers rapidly so as to protect the fuel kernels from abrasion effects. The diffusion of fuel can be minimized by coating at temperatures below those at which diffusion becomes significant, or alternatively by coating at high temperatures or methane concentrations so that the rate of deposition exceeds the rate ‘of diffusion.
FIG. 23.~Sintered zirconium-uranium monocarbide particle with interrupted laminar pyrolytic carbon, silicon carbide and interrupted laminar pyrolytic carbon coatings deposited at 1500°C. [X 1251
FIG.
24.-Porosity
or inclusions in layers of silicon carbide deposited with insufficient partial pressure of hydrogen. [ x 1251 638
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
639
8.1 Thorium-uranium dicarbidefuel Early results for the surface contamination (as determined by a-particle counting) of pyrolytic carbon coated kernels of thorium-uranium dicarbide showed such 6000 Kernel material: Natural thorium-uranium dicarbide x Laminar structure o Transitional structure v Columnar structure @Coating thickness (~1
5ooo
;@ I I I
I
4000.-
I
m :: 2 3000.. v) E z 0 A
I
zooo--
I
iooo-
0
FIG.
1400 1600 moo Temperature of deposition,
2S.-Dependence
2000 V
of a-count on temperature of deposition.
wide scatter that it was impossible to determine the effect of temperature
and rate of deposition of the pyrocarbon, or of the structure of the coating; or of kernel parameters such as alloying ratio, enrichment and physical form. Furthermore, the surface contamination was frequently affected by the formation of products of hydrolysis of thorium-contain@ carbide fuel, proving that the pyrolytic carbon was porous. Distinctive porosity has been observed at the junctions between adjacent cones of the columnar structure. A full understanding of the mechanism of the effect is not yet available but fuel coated with the more recently developed pyrocarbons or with an interlayer of silicon carbide is not susceptible to hydrolysis. The basic reasons for the relatively high a-activity of pyrolytic carbon coated thorium-uranium dicarbide fuel have been postulated by POINTUD(1964, private communication) from examination of the evidence available. He has pointed out that the thorium metal powder originally used in the preparation of fuel particles was characterized by the presence of highly active daughter products, e.g. 2MRa, in proportions depending on their half-lives and the period of storage since initial 2
640
J. R. C. Gouo~ and D. ,KERN
production of the powder. The radium can be expected to be volatilized at the high temperatures involved in the calcium reduction of thoria, and again in any subsequent high temperature heat treatment prior to the coating operation. Any radium present at the time of coating can be expected to diffuse rapidly through the pyrolytic carbon in a manner similar to barium or strontium. Therefore the a-activity of pyrolytic carbon coated thorium-uranium dicarbide fuel particles depends primarily on the time interval between completion of sintering of the kernels and completion of the coating operation, and on the relative rates of diffusion and carbon deposition. Experimental results from the coating of thorium-uranium dicarbide fuel with pyrolytic carbon have supplied supporting evidence. For example, the a-activity of a 1600°C laminar coating of thickness 80 p increased by a factor of 9 as the time interval from the end of heat treatment to the start of coating was increased from 90 min to one week. Measurements at various intervals of time after coating showed that the radioactive decay was characteristic of the daughter products of thorium. 8.1.1 E@ct of variations in the values of the coatingparameters. A study was made of the effects of variation in coating conditions on the extent of surface contamination of coated thorium-uranium dicarbide fuel. All fuel kernel batches were heat treated at 2200°C in vacua for 2 hr prior to coating in order to volatilize the highly a-active daughter of thorium. Figure 25 shows that for a constant methane concentration, the surface contamination increased linearly with temperature of deposition. For deposition temperatures above 1450°C the least contaminated coating was the metallographically laminar or structureless type deposited at a rate of >50 p/hr using a methane concentration of 30 per cent. For lower deposition rates ((20 p/hr for transitional and
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
641
of halogens or halides to form and remove volatile compounds of uranium and thorium. Carbon tetrachloride vapour has the advantage that pyrolytic carbon is deposited concurrently in such a manner as to seal the surface of the coating. 15 ,u of pyrolytic carbon deposited from carbon tetrachloride vapour reduced the contamination of the laminar and columnar metallographic structures by about one order of magnitude ; a low level of contamination was obtained even with fuel kernels containing enriched uranium; and the improvement was comparable with the effect of an interlayer of silicon carbide. Pyrocarbon deposited from carbon tetrachloride vapour has several distinct advantages over layers of silicon carbide, which include a lower neutron absorption cross section, elimination of the hazards associated with the use of hydrogen, and simplification of chemical recycling processes. 9. CONCLUSIONS
The metallographic structure of pyrolytic carbon coatings on fuel kernels is sensitive to the conditions of fluidization, the concentration of hydrocarbon and the temperature of deposition. Temperature gradients in a laboratory-scale fluidized bed of fuel particles increase with the density of the particles and to a lesser extent with gas flow, but decrease with increased weight of charge. A modified graphite fluidization nozzle considerably reduced the temperature gradients. The carbon efficiency of the deposition process increases as deposition proceeds, probably as a result of the greater available surface area and changing conditions of fluidization. At constant coating temperature the carbon efficiency varies linearly with methane concentration, increasing in the zone of laminar deposition but decreasing in the zone of columnar deposition. This leads to a situation in which at a given temperature higher carbon efficiencies can be obtained with decreasing methane concentration, provided that the change in methane concentration is such as to change the metallographic structure of the deposited pyrocarbon. The rate of carbon deposition increases with the density of fuel kernels being coated, for constant weight of charge and within the range of coating parameters investigated. The rate of deposition at a given temperature was found to increase linearly with methane concentration, within each of two separate ranges of concentration corresponding to the different metallographic structures. An increase with temperature was observed for all methane concentrations, although previous data for the higher methane concentrations has shown that the rate of deposition reaches a maximum value and thereafter decreases with further temperature increase. The temperature giving the maximum rate of deposition was lower for the higher methane concentrations. The room temperature density values of vapour deposited pyrolytic carbons determined in the present studies show the characteristic minimum when plotted against temperature of deposition, although the temperatures of deposition 16001700°C giving the minimum density values are higher than those reported elsewhere. It is not the intention in this paper to discuss the basic reasons for the density minimum but it should be mentioned that the several contributory factors which have been suggested include the deviation of the apparent c-spacing from the ideal value, porosity formed by anisotropic thermal contraction from the deposition temperature, and very fine porosity detectable by electron microscopy. In addition, pore space
642
J. R. C. GOUGHand D. KERN
could be occupied by incompletely pyrolysed hydrocarbons. Other hypotheses to account for the density minimum are based on the mechanism of deposition of the pyrolytic carbon and the surface mobility of the deposited atoms or radicals at various temperatures. Interlayers of dense j3-silicon carbide, which are known to have favourable retention characteristics for certain metallic fission products are deposited from methyltrichlorosilane vapour in hydrogen carrier at a typical temperature of 1500°C. At higher temperatures or with insufficient partial pressure of hydrogen the apparent quality of the deposited silicon carbide deteriorates by the co-deposition of a-silicon carbide and of carbon. Coatings on fuel particles are assessed as regards metallographic structure, density, crushing strength, contamination in the surface layers as measured by acounting, and finally fission product release. The original metallographic classification of pyrolytic carbon as ‘laminar’, ‘structureless’ or ‘columnar’ is however being increasingly replaced by a more basic, structural classification in which the isotropy and crystallite size are determined as well as the density. The surface contamination of pyrolytic carbon-coated fuel particles depends, inter alia, on the rates of deposition of the pyrocarbon and diffusion of the fuel at the temperature of coating. Initially results showed an increase of surface contamination with temperature of coating within the range 1400-19OO”C,the increase with temperature being most rapid for the low rates of deposition giving the columnar structure. Recently however it has been shown that the very high rates of deposition possible using high methane concentrations at temperatures above 2000°C can result in surface contamination of a very low order. The incorporation of silicon carbide interlayers, and leaching with carbon tetrachloride vapour (which also results in the deposition of pyrolytic carbon) have been shown to give surface contamination often close to background level. Acknowledgments--The authors’ thanks are recorded to Mr. R. A. U. HUDDLE,Head of Materials Branch of the DRAGON Project,for his continual interest and guidance, to Mr. R. BICKERDIKE, Mr. H. BE~TLERand Mr. C. VIVANTE for useful discussions, and to Mr. C. A. RENNIE,Chief Executive of the DRAGON Project,for permission to publish this paper. REFERENCES B~KROSJ. C. (1964) The Structure of Pyrolytic Carbon Deposited in a FluidisedBed, Report GA-5163. Bormos J. C. and PRICER. J. (1965) Radiation-Induced Dimensional Changes in PyroIytic Carbons Deposited in a Fluidised Bed, Report GA-6736 CARL.EY-MACAULEY K. W. and MACKENZIE M. (1963) Studies on the Deposition of Pyrolytic Carbon, Proceedings of the Fzyth Carbon Conference, Vol. 2, p. 449, Pergamon Press, Oxford. DIEP~NWRPR. J. (196O)J. chim. Phys. 57,815. R. H., PAT~SONJ., P IJMMERY F. C. W. and W~rrs R. E. (1962) Some Measurements of FLOW the Diffusion of =U Fission Products and =Pa from ‘Coated ‘Fuels, Report AERE-R 4215. NORDWALL H. J. DE(1967) Private communication. POINTUDR. (1964). Private co~unicatiOn. Prwx M. S. T., G~UGH J. R C. and HORSLEY G. W. (1966)J. Br. Nucl. Energy Sot. 5,360. Proceedings of the Symposium on Ceramic Matrix Fuels Containing Coated Particles (1962) Battelle Memorial Institute, Columbus, Ohio, U.S.A., November 1962, TID/7654. SAYERS J. B., ROSEK. S. B., C!ooBSJ. H., HAUSER G. P. and VIVANTE C. (1963) Carbides in Nuclear Energy, Vol. 2, p. 919, Macmillan, London.
Pergamott Press Ltd.
Printed in Northern Ireland
STUDIES ON THE COATING OF FUEL PARTICLES FOR THE ‘DRAGON’ REACTOR EXPERIMENT J. R. C. GOUGH* and D. KERNf (First received 4 January 1967 and injinalform
1 March 1967)
Abstract-The requirements of fission product retaining coatings on coated fuel particles are reviewed and the scope of development necessary to meet these requirements for the initial charges of the DRAGON Reactor Experiment is described. These studies on the coating of fuel particles were carried out using a laboratory scale fluidized bed type of coating furnace. Within the range of deposition temperatures investigated, laminar or metallographically structureless pyrolytic carbon was deposited with methane concentrations greater than 15 per cent, while the structure deposited at low methane concentrations was columnar. Appreciable temperature gradients were found in these small fluid&d beds of fuel particles, particularly under the relatively static conditions of fluidization which favoured deposition of the columnar structure. The rate of deposition of pyrolytic carbon was found to increase linearly with methane concentration at a given temperature, and within specified limits of methane concentration such that the relationship was linear within each of two separate ranges of concentration corresponding to the different metallographic structures. The room temperature density of the deposited pyrocarbons showed minimum values for deposition temperatures of 1600-1700°C. Under the experimental conditions described, the carbon efficiency of deposition increased to the order of 100 per cent at deposition temperatures of 1600°C and above. The deposition of silicon carbide interlayers from methyltrichlorosilane is described. The surface contamination of coated particle fuel was shown to be sensitive to the temperature of deposition of the pyrocarbon, to the rate of deposition at a fIxed temperature, and to the presence of silicon carbide interlayers. The latter reduced surface contamination to near background values. 1. INTRODUCTION
THE development of high-temperature gas-cooled reactors depends to a large extent on the integrity of the fission product retaining coatings on coated fuel particles. The fuel particle coatings are required to fulfil the following functions: (a) To facilitate fuel processing and assembly, not only by reducing contamination levels but also by preventing the hydrolysis which would otherwise occur in the case of thorium containing carbide fuel. (b) To protect the fuel from reaction with impurities in the coolant or with matrix materials. (c) To prevent or reduce to a minimum, migration or decomposition of the fuel. (d) To delay or retain gaseous and solid fission products to high burn-up levels in order to facilitate maintenance of the primary circuit, to reduce the load on or to eliminate entirely the fission product clean-up plant, and to reduce shielding
requirements.
The primary development of fuel particles coated with pyrolytic carbon and silicon carbide for the DRAGONReactor Experiment has been described in a paper presented at the Battelle Symposium on Ceramic Matrix Fuels Containing Coated Particles (1962). Fuel element fabrication for the reactor experiment was described in a paper (PRICE et al., 1966) presented at the British Nuclear Energy Society Symposium on High-Temperature Reactors and the DRAGONProject. The purpose * DRAGONProject, APE, Winfrith, Dorchester, Dorset. t Deceased, previously DRAGON Project. 623 1
624
J. R. C. Goucm and D. KERN
of the present paper is to give the results of studies on the coating of fuel particles carried out in the period up to the fabrication of the initial fuel charges for DRAGON. 2. REQUIREMENTS
OF THE
COATING
The many requirements to be met by the coatings on fuel particles range from those affecting the fabrication processes to those related to irradiation performance. It is desirable for example that any vapour deposited coating can be applied at a temperature above that of reactor operation so that differential expansion will not ‘&use fracture or prestressing of the coating. Similarly the thermal expansion of the coating should be not greater than that of the fuel kernel, otherwise the coating will tend to fracture on cooling from the coating temperature, as in the case of MgO deposited on to dense UO,. However, differential expansion effects may be alleviated by porous, spongy interlayers. The coating should be free from deleterious impurities, for example, adsorbed or occluded hydrogen, which might increase the neutron absorption cross section or react chemically with the fuel or the coating. However, most gaseous and volatile impurities are readily removed by vacuum heat treatment which is normally carried out after consolidation of the fuel, and has the additional objective of stabilising the dimensions of the fuel compacts. Additionally, the coating should be relatively free from fuel contamination originating from the kernel, in order that the release of fission products shall be minimized. Such contamination can arise from thermal shock and/or self-abrasion of fuel kernels in the early stages of the coating procedure, followed by incorporation of the fuel dust in the deposited pyrocarbon. A further possible cause of the formation of fuel dust is chemical reaction between the fuel kernels and impurities either in glove box controlled atmospheres, or in the carrier and coating gases in the fluidized bed or tumbling furnace used to deposit the coatings. Contamination of coatings can also be caused by fuel diffusion from the kernel during the actual coating operation. The diffusion of uranium, for example, through pyrocarbon as deposited in the early stages of development became significant at coating temperatures of 1550°C and above, while the radium-228 and radium-224 daughter products of thorium diffuse rapidly at even lower temperatures. Dispersion of the coated particle fuel in a graphite matrix requires that the coating should have sufficient mechanical strength to withstand the processes of mixing and subsequent pressing or extrusion. The proportion of coatings whose breakage can be tolerated will depend in any particular application on the fission product retention properties of the kernel itself and of the matrix, whether fission product release is controlled by the use of a purge flow, and so on. However, the incidence of fracture of coatings can be minimized by appropriate modification or special development of techniques, including for example, ‘overcoating’ with matrix material prior to consolidation. The physical and chemical properties of the coating must satisfy many requirements. For example, the coating should be compatible with the fuel, matrix and coolant under operational conditions. It should have a high melting point or decomposition temperature, low volatility, show no phase changes up to maximum operating temperatures, and must maintain its integrity under fast neutron irradiation and thermal cycling. The neutron absorption cross section should be acceptably low.
FIG. I.-Zirconium-uranium
monocarbide fuel for the first charge of the Reactor Experiment. [x 1251
FIG. 2.-Thorium-uranium
dicarbide fuel for the first charge of the Experiment. [X 1251
DRAGON
DRAGON
Reactor
624
FIG. 3.-Laboratory
scale fluidizing furnace.
FIG. 5.-Duplex
FIG. 6.-Sintered
pyrolytic carbon coated fuel particle showing laminar and columnar structures. [ X 2001
thorium-uranium dicarbide coated with two layers of laminar pyrolytic carbon at 1600 and 1900°C. [x 1251
FIG. 7.-Enriched melted thorium-uranium dicarbide particles with ‘interrupted laminar’ pyrolytic carbon coating deposited at 1500°C. [ x 1251
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
625
In order to achieve good retention properties for gaseous and volatile fission products the coating is required to be of low permeability. The diffusion coefficients for fuel and solid and gaseous fission products should be low under service conditions, and in this respect the small lattice parameter of silicon carbide is particularly attractive. The selection of the optimum density of coating layers is dominated by the fact that high density and an isotropic structure are required to minimize fast neutron irradiation induced shrinkage (BOKROSand PRICE,1965) while porous inner layers are desirable to accommodate fission product gases and the swelling of fuel kernels. These requirements and the need to delay the diffusion of fuel and fission products have led to the development of duplex and triplex coatings giving the optimum combination of properties for fission product retention. Finally it is essential that the coating material should be economic not only from the point of view of initial fabrication, but also in that it should not adversely affect the economical reprocessing of the fuel. 3. SCOPE
OF DEVELOPMENT
The first charge of fuel elements for the DRAGON Reactor Experiment was arranged as a two-zone core, the inner zone containing fuel in the form of thorium-uranium dicarbide while in the outer annulus the fuel was zirconium-uranium monocarbide. In both cases the coated fuel particles were dispersed in a matrix consisting substantially of graphite. The total fissile loading of 14 kg of 2W at an enrichment of 93 per cent by weight was distributed uniformly throughout the core. The thorium ratio in the inner zone was Th: 235U= 10: 1 by atoms, while the zirconium ratio in the outer zone was Zr: 235U= 8 : 1 by atoms. The zirconium-uranium monocarbide fuel particles were of nominal diameter 251-422 p and were coated with two layers of pyrolytic carbon, the inner being deposited at 1600°C and the outer at 1450°C (Fig. 1). In the case of the thoriumuranium dicarbide fuel particles the kernel diameter was nominally 353-500 ,u and the triplex coating consisted of an inner layer of pyrolytic carbon deposited at 15OO”C, an intermediate layer of silicon carbide also deposited at 1500°C and an outer layer of pyrolytic carbon deposited at 1450°C (Fig. 2). In the second charge of the Reactor Experiment the zirconium-uranium monocarbide fuel has been replaced by fuel consisting of a dispersion of uranium dicarbide in excess carbon. The latter kernels are of diameter 420-572 ,u and are coated with pyrolytic carbon and a 35 ,u interlayer of silicon carbide to give fuel particles of 710-1000 ,LJ overall diameter. The pyrolytic carbon coatings were deposited at higher temperatures than in the case of the first charge, and were therefore of higher density and more isotropic. A number of advanced thorium-uranium carbide and oxide fuels were also developed and are being irradiated in the second charge. Plutonium-bearing coated particle fuel is also being evaluated by irradiation in the second charge of the Reactor Experiment. The early development of coating technology within the DRAGON Project had as its immediate objective the coating of small batches (<40 g) of fuel particles with pyrolytic carbon for irradiation experiments. The first fuel particles to become available were porous, sintered uranium dicarbide of diameter 144-251 p and the
626
J. R. C. GOUGHand D. KERN
coating deposited was plain pyrolytic carbon. Subsequent development resulted in the production of fuel kernels of zirconium-uranium monocarbide and thoriumuranium dicarbide with zirconium : uranium and thorium : uranium ratios of up to 8 : 1 and 15 : 1 by atoms respectively. These had a kernel diameter of 251-500 p and were coated either as sintered (mercury density 3.5-6.5 g cm-3) or as melted (mercury density 6-8 g crnA) with pyrolytic carbon, and in some cases with an interlayer of silicon carbide. More recent developments have included the preparation of 353572 ,u kernels of uranium dicarbide with a large excess of carbon, a tendency to increase kernel diameters to satisfy heavy metal concentration requirements, the development of plutonium bearing fuels, and a progressive increase in the temperature of deposition of pyrocarbon coatings. The selection of optimum temperatures of deposition and concentrations of hydrocarbon has been shown (BOKFIOSand PRICE) to give pyrocarbons having favourable values of structural parameters (density, isotropy and crystallite size) to minimize fast neutron irradiation induced shrinkage. 0
Argon Feeding
purge
Feeding container
tub
Fluidizing
odiation
FIG. 4.-Schematic
vessel
shields
diagram of laboratory-scale fluidtig
furnace.
The purpose of the DRAGON fuel irradiation programme has been to confirm that the fuel selected for the tist and subsequent reactor charges will withstand the service conditions of burn-up and temperature. These objectives have involved the development of appropriate coating procedures and detailed investigations into the rBle of the various coating parameters. All the information presented in this paper has been obtained using the laboratory scale fluidized bed type of coating furnace shown in Figs. 3 and 4. Variations in the concentration of hydrocarbon were known to give rise to a distinctive change of metallographic structure from laminar to columnar as shown in Fig. 5. One of the first objectives of the fuel irradiation programme was to compare the fission product retention properties of these two structures alone or in combination, deposited at various temperatures in the range 1450-1900°C. Another objective was
Studies on the coating of fuel particles for the DRAGON reactor experiment
627
to investigate whether discontinuities in the deposited coating in the form of a change of structure, a gap, or an interlayer of for example silicon carbide, improved the irradiation performance of the fuel by acting as effective barriers to the diffusion of fuel and fission products. Under certain conditions of coating, pyrolytic carbon tends to be deposited within the pores of sintered kernels rather than as a layer around the kernel. As this could significantly affect the amount of free volume available within the kernel for the accommodation of gaseous and volatile fission products, it was necessary to determine the extent of this effect. In the case of pyrolytic carbon the coating variables of principal interest are the temperature of deposition, which affects the density of the coating and the rate of deposition; thehydrocarbonconcentration, which affects the metallographic structure, the rate of deposition and to some extent density; and the time of coating. The dependence of carbon efficiency, rate of deposition and density of the pyrocarbon coating on these variables and on the weight and density of the charge have been studied in detail. However, carbon efficiency and rate of deposition in laboratory-scale fluidized beds are markedly dependent on temperature distributions. The latter were therefore investigated as a function of gas flow, weight of charge and the density of the fuel particles. 4. DEPOSITION
OF PYROLYTIC
CARBON OF VARIOUS STRUCTURES
4.1 Coating agents Pyrolytic carbon has been deposited on fuel particles in the DRAGONProject laboratories at optically measured temperatures between 1400°C and 1900°C (and recently at temperatures well in excess of 2OOO”C),by vapour decomposition of hydrocarbon gases. The large majority of experiments were carried out using methane as coating agent, but propane, butane and acetylene were investigated as alternative sources of pyrolytic carbons particularly when porous deposits were required. Carbon tetrachloride vapour has been used as a gas leaching agent to reduce the surface contamination of coated particle fuel by the formation and removal of volatile halides of uranium and thorium; during this leaching process pyrolytic carbon was formed as an outer sealing layer at a very low rate of deposition. Table 1 gives typical compositions of the argon carrier gas and of the methane, the latter being used as coating agent in the large majority of experiments. The impurity contents of carrier gases and coating agents are recognized as possible causes of reaction with kernel materials and hence pollution of the vapour deposited coating TABLE l.-TYPICAL COMposll-IONSOF ARGGN CARRIERGAS AM) OF METHANEUSED IN THE COATINGOF FUEL PARTICLES
Hydrogen Argon
(99.995 “/,I Mkthane -. (85.6 %>
41 v-pm
-
Nitrogen
Carbon dioxide
Carbonaceous matter determined as carbon dioxide
15vpm
-
lvpm
lvpm
150 vpm
10.32%
0.94 %
-
Oxygen
Water vapour
2vpm 3.14%
628
J. R. C. Gouorr and D. KERN
with radioactive debris. To minimize the influence of moisture, coating and carrier gases were dried by passage through a column of molecular sieves and by this means moisture levels of 4-6 vpm were obtained. 4.2 The laminar structure of pyrolytic carbon In accordance with the results of other investigations in this field (Proceedings of BOKROS,1964) the metallographic appearance of pyrocarbons deposited onto fuel particles in the temperature range 1400-1900°C was laminar or structureless when the methane concentration was 15-50 per cent of the total gas flow. The distinctively laminar metallographic structure is characterized by the presence of circumferential rings or laminations which may be caused by fluctuations of temperature or methane concentration experienced by individual fuel particles in the fluidized bed. The effect is probably enhanced by relatively slow circulation of the fuel particles, particularly in the case of deep beds. With the coating conditions used in the present investigation, however, the pyrolytic carbons deposited have normally been found to show no apparent metallographic structure (Fig. 6). the Symposium on Ceramic Matrix Fuels Containing Coated Particles;
4.3 The columnar structure of pyrolytic carbon It is well known that the pyrolytic carbon obtained by deposition on to free surfaces of for example heated rods, or furnace walls, has a characteristic cone structure (CARLEY-MACAULEY and MACKENZIE,1963). A similar structure is shown when pyrolytic carbon coatings are deposited on to fuel particles using tumbling furnace techniques. In the course of the development of the technology of coating fuel particles within the DRAGONProject, investigations have been carried out to define the conditions under which pyrolytic carbon coatings of columnar structure are obtained using laboratory-scale fluidized bed furnaces. Deposition of this structure has been found to be favoured by relatively static conditions of fluidization, for example at low gas flows, in fluidizing reactors of wide cone angle, and when fuel kernels of high density and/or large diameter are fluidized. 4.4 Pyrolytic carbon with intermediate discontinuities In the DRAGONfuel irradiation programme, the inner layer of pyrolytic carbon deposited on to fuel particles has sometimes contained deliberately introduced circumferential discontinuities which are intended to act as barriers to the diffusion of fuel and fission products and to prevent the propagation of cracks through the coating. Several techniques investigated to obtain these discontinuities are described below. On the basis of previous experience of the deposition of pyrolytic carbon of various structures, a technique has been developed which consists of decreasing the hydrocarbon concentration from that used for the laminar structure to that for the columnar structure for an interval sufficient to give a zone 2-3 p thick, followed by reversion to the conditions for a laminar structure (Fig. 7). Several techniques found to be unsuccessful included cooling partially coated particles within the furnace followed by reheating to the coating temperature; interruption of the flow of methane for a short period; increasing the methane
Studies on the coating of fuel particles for the DRAGON
reactor experiment
629
concentration to a high value for a short period; and cooling the particles down to 77°K after removal from the fluidizing bed in order to achieve an interruption by thermal contraction of the kernel. Previous experience had shown that circumferential discontinuities sometimes form in the coating close to the kernel during cooling from coating temperatures to ambient, but that this only occurs in the case of sintered kernels of relatively high density; the effect can therefore vary from fuel particle to fuel particle in the same coating batch. Removal of partially coated fuel into the laboratory atmosphere followed by continuation of the coating process was partially successful but did not give reproducible results. 5. TEMPERATURE
DISTRIBUTIONS SCALE FLUIDIZED
IN LABORATORY BEDS
Although the movement of fuel particles inside fluidized bed furnaces has been studied by carrying out model experiments at room temperature using higher gas flows to simulate the conditions at normal deposition temperature, it has been difficult to draw valid conclusions from the results obtained because of the major influence of temperature in fluidization techniques. Also, in order to obtain detailed information on the kinetics of the deposition of pyrolytic carbon on fuel particles, an essential preliminary was the measurement of temperature distributions inside the fluidixed bed reactor tube. A systematic investigation was carried out on the effects of density of fuel particles, weight of charge and flow rate of carrier gas on temperature distributions in laboratory-scale coating furnaces of this type. A uniform distribution is desirable from the point of view of consistency of the product; furthermore the thermal cycling inherent in some coating processes may have an influence on fission product retention properties.
5.1 Experimentalresults In the temperature range investigated the total gas flow had only slight influence on the temperature distribution, other conditions being constant (Fig. 8). The depth of the bed of fuel particles and the uniformity of temperature distribution increased with weight of charge (Fig. 9). The density of the fuel particles had a marked influence on the pattern of temperature distribution, the average particle temperature decreasing with increase in particle density. However, this effect was less pronounced in the case of 40 g charges, (Figs. 10 and 11). The maximum temperature gradients in the bed of fuel particles were found under conditions normally used for the deposition of columnar pyrolytic carbon coatings, that is at low gas flows in fluidizing reactors of wide cone angle. These gradients measured by traversing a thermocouple through the fluidized bed of fuel particles were compared with the temperature measured optically at a fixed position on the heater tube as shown in Fig. 4. Under conditions simulating the deposition of columnar pyrolytic carbon the average fuel particle temperature was 310°C below that measured optically. A 50 per cent increase in gas flow decreased this difference by only 45°C (Fig. 12). In view of the experimental observations on temperature gradients in the laboratoryscale fluidized bed it was desirable to obtain a more uniform particle temperature over the whole range of the fluidized bed depth. By this means the temperature peak which is pronounced for 20 g charges and also evident for 40 g charges would be obviated. The requirements to preheat the carrier gas and to avoid excessive loss
J. R. C. &NJOH and D. KJJRN
630
160IO -
I
140 10 -
Y 120 d 3 0 ; z c”
.
IOC IO
8C IO
Temperature: Maferial: 2Og X No particles 0 200 MO,C . 2Og M&C v
200
1460*C prccoalcd Mo$ (p=b59g/cm3) flow 1515 cm3/min flow ISIS cdlmin flow 2035 cm3hnin
MO&
flow
2555
I
cn?/min
I 6C 10
I
0
Distance
FIG. I.-Temperature
800-
30
20
IO
from
nozzle,
cm
distribution of the fluidized bed fitted with a normal graphite nozzle (28’) at various flows of carrier gas.
Tempwofurc: l460.C Gas flow: I515 cdfmln X No porticlcs 0 200 Mozt (p=2.59g/cm3) v 409 Mo,C (p=2.59g/cm31
Distance
from
nozzle,
cm
FIG. 9.-Influence of charge on the temperature distribution of the fluid&d bed.
Studies on the coating of fuel particles for the DRAGONreactor experiment
160
140
Y
120
90
Temperature: 1460.C Gas flow: 1515 cm%nin Charge: 209 + No particles x MO& p=259q/cm3(12~5cm3) 0 tZr, UtC 7.44: lip=5.37g/cm3 v
uCz
(3M)
60
,p=t0.45g/cm3
FIG. lo.-Infkence
cm’1
! 20
IO Distance
t6~9cdt (3.0
nozzle,
from
‘;
‘30
i-
cm
of particle density on the temperature bed (20 g charge).
distribution
of the fluidized
+ 5 a
1600
1400
‘x \
\
y7
‘+ \
’\
’\
“\\
___
“I
8OC
Temperature: l460.C Gas flow: 1515 cd/min Chorqe: 400 + No parlicles X Mo*C. p=2,59g/cm3 0 (Zr, WC, 7.44: Ii p=5.37g/cd v ucz
600
(3M)
p=IO~45~/cm3
IO Distance
FIG. 11.-Influence
20 from
nozzle,
cm
of particle density on the temperature bed (40 g charge).
distribution
of the fluidked
631
J. R. C. OouoEI and D.
632
Distance FIG.
from nozzle,
cm
12.-Temperature distribution of the fluidized bed using conditions suitable for depositing columnat pyrolytic carbon layers.
Distance FIG.
KERN
13.-Inlluence
from lube inlcl,
cm
of charge on the temperature distribution of the fluidized bed using the improved nozzle.
Studies on the coating of fuel particles for the
I
I
DRAGON
I J.
I
I
reactor experiment
5 IO I5 20 25 Distance from nozzle inlet, cm
Distance
FIG. 14.-Comparison
Y
1700
m- 1600 Z! 0
from
nozzle
inlet,
cm
of the two nozzle systems to determine the influence of particle density on the temperature distribution.
Temperature: 1460. -- -
409 charge Improved no~zle:209charge 406 charge
z g I500 'ij 2 1400 3 ? $I300 E
1%
2 % 1200
-_
’ 1.
._
!! u a
-_ -.
IlOO
5
4
5 Particle
6
7 density,
6 g/cm’
9
IO
II
FIG.15.--The influence of particle density on the average particle temperature.
633
634
J. R. C. GGUGH
and D. Ih3RN
of heat to the water-cooled gas injection assembly led to a graphite nozzle design whose principal features were an extended oril%e between the gas injection assembly and the conical base of the fluidizing reactor and the incorporation of a replaceable inner sleeve. The new design gave a signilicantly more uniform temperature distribution of the fuel particles; however, a slight temperature peak was still observed in the case of 20 g batches (Fig. 13). The temperature of the experiment within the range investigated had little or no influence on the pattern of temperature distribution. Temperature distributions were much less affected by changes in the density of fuel particles in the case of the improved nozzle than they were for the previous nozzle (Fig. 14). This point is further illustrated in Fig. 15. 6. INFLUENCE OF THE COATING VARIABLES ON THE DEPOSITION OF PYROLYTIC CARBON
6.1 Coating thickness The sintered particles produced by the DRAGON powder agglomeration process have sometimes been characterized by sufficient open porosity to allow carbon to be deposited within the kernel during the initial stages of the coating process. A series of experiments were carried out to make a quantitative estimate of the amount of carbon deposited into typical porous kernels at 1600°C. The results obtained (Fig. 16) indicate that during the initial 5 min of coating, 12 wt. % or 30 vol. % carbon is deposited into these kernels. The relationship between coating thickness and weight increase at 1500°C is similar to that at 1600°C except that the coatings corresponding to a given weight increase are thinner. This observation is in agreement with density measurements, which show minimum values when the pyrocarbons are deposited between 1600°C and 1700°C. 6.2 Rate of deposition The rate of deposition of pyrolytic carbon on to fuel particles increases considerably with increase in fuel particle density (the weight of charge being constant), as the same weight of deposited carbon is distributed amongst a smaller number of particles. The dependence of the rate of deposition on the concentration of methane at a coating temperature of 1600°C is linear as shown in Fig. 17. The relationship between the rate of deposition and the reciprocal of the absolute temperature of coating is also linear (Fig. 18). In the case of 4.2 per cent methane concentration and 15 g charges, a 90” included angle graphite nozzle and a total gas flow of only 670 cm3 min-1 (instead of 1520 cm3 min-l) were used in order to obtain the relatively static conditions required to achieve columnar deposition. 6.3 Coating density The room temperature density of vapour deposited pyrolytic carbon of both the laminar and columnar metallographic structures varies with the temperature of deposition and shows maximum values -2-O g cm-s for temperatures of 1000°C and below, and 1800°C and above. Minimum density values of about l-75 g cm-3 at 1600°C were reported in the case of the columnar structure and l-5 g cm-3 at 1450°C in the case of the laminar structure (Proceedings of the Symposium on Ceramic
Studies on the coating of fuel particles for the 60-
I
Totol flow 1470cm’Imin Charge 13.5 g methone
50
I t $a
concentrotionz50%
40
IO
20
Coating
FIG. 16.-Variation
j_
30
time,
min
of weight increase and coating thickness with time of coating 1600°C for (Zr, U)C/8 : 1 sintered particles.
Material: Charge:
e
/
/
0
.iii 0
Zirconium-uranium 13.59
monocarbide
40-
n
0
IO
I 30
20
Methone FIG.
17.-Dependence
Material: -
4.8
1900
FIG. 18.-Variation
.
Thorium-uronium 29.3% Methone 10.9% Methane 4.2% Methone
I
I
5-o t
t
1900
- - -
I
I
I
46
40
50
%
concentration.
of deposition rate at 1600°C on methane concentration.
-
0
635
reactor experiment
DRAGON
5.2 t 1700
5.4 t I600
dicarbide
concentration concentration concentration
20g 209 l5g
charge charge charge
I
1
56
t I500
of deposition rate with temperature
5.8 t 1450
I/OK x IO4
OC
(l/K).
at
636
J. R. C. GOUGH and D.
I "'800
Fro. lg.-Variation
I 1000
I 1200 Deposition
I 1400 temperature,
KERN
I 1600 T
I I800
2‘0130
of density of pyrolytic carbon coatings on fuel particles with temperature of deposition.
Matrix Fuels Containing Coated Particles). These results are plotted in Fig. 19 in conjunction with density values measured in the course of the present investigations. Density values were determined by crushing the coated particles, leaching away the kernel material and measuring the density of coating fragments by the ‘sink and float’ method using liquids of standard density. It can be seen from Fig. 19 that under the conditions of the present laboratory scale coating experiments, the temperatures of deposition giving the minimum values of coating density are in the range of MO1700°C and that the methane concentration and therefore rate of deposition appears to have no pronounced i&rence on coating density. However there is a tendency for the coatings deposited at a low rate of deposition to show slightly higher density values, especially at the maximum temperature of deposition, which was 1900°C in this series of experiments. DIEFE~RF (1960) has shown that the characteristic minimum in the curve of coating density against temperature of deposition becomes less pronounced with decrease of pressure and finally disappears when the coating is applied under very much reduced pressure, e.g. 1.7 x low2torr.
6.4 Carbon ejiciency The dependence of the carbon efficiency of deposition (defined as the ratio of carbon deposited on the fuel particles to that contained in the coating agent) on the time of coating and on the methane concentration is illustrated in Figs. 20 and 21 respectively. The influence of the temperature of deposition on the carbon efficiency for the laminar, fine-grained transitional, and columnar metallographic structures is shown in Fig. 22, from which it is apparent that for any given temperature of deposition, higher carbon efficiencies are obtained with decreasing concentrations of
Studies on the coating of fuel particlti for the
100
.o
t
Material:
Zirconium-uranium
DRAGON
monocarbide
35.8%
u 0
IO
Fro. 20.-Variation
1 20
reactor experiment
I
I
I
30 Time.
melhone
50
40 min
I
60
‘r
of carbon efkiency with time of coating for various methane concentrations at 16WC.
Materiot: Zirconium-uranium 13.59 charge
monocarbide
J
zo-
I IO
0 FIG.
21.-Influence
I
20 Methane
I
40
I
SO’
%
of metbane concentration on carbon efficiency for laminar and columnar coating structures deposited at 1600°C.
Temperature, FIG.
I
30 concentration,
22.4nfluence
‘c
of temperature on carbon efficiency.
637
638
J. R. C. GOUOH and D. KERN
methane. Although relatively low carbon efficiencies (of the order of 30-40 per cent) have been observed at coating temperatures of 1450°C and below, the values for each metallographic structure of pyrolytic carbon increased to the order of 100 per cent at 1600°C and above. 7.
THE
DEPOSITION
OF
SILICON
CARBIDE
AT
1500°C
Although relatively volatile carbide-forming fission products such as strontium, barium and caesium are not retained by pyrocarbon coatings to the same degree as the rare gases (FLOWERS et al., 1962), it has been shown that their release is considerably reduced by the incorporation of interlayers of silicon carbide (DE NORDWALL, 1967; private communication), Techniques for the application of silicon carbide coatings were therefore developed by the DRAGON Project and fuel with silicon carbide as an intermediate coating layer was tested in various irradiation experiments (SAYERS et al., 1963). Silicon carbide deposited at 1470°C from methyltrichlorosilane vapour in a hydrogen-argon mixture was dense and of uniform thickness with a smooth outer surface. Increasing the temperature of deposition to 1670°C resulted in apparent porosity and a distinctly rougher outer surface although the inner showed little if any change. For these reasons a temperature of 1500°C was chosen as the deposition temperature for silicon carbide coatings in irradiation specimens. Using a hydrogen carrier concentration of 50 per cent smooth layers could be obtained, which were not interlocked with the adjacent pyrocarbon layers (Fig. 23). With decreasing concentration of hydrogen in the feed gas mixtures, progressive deterioration in the quality of the SIC layer occurred as for example in Fig. 24 showing coatings obtained using only 235 per cent hydrogen. In these experiments only the proportion of hydrogen in the silane carrier gas flow was varied, the total flow of argon and hydrogen through the silane being kept constant. With particles in a density range of 3.54~5 g cm-s deposition rates of 30-50 p/hr were obtained using 40-50 g charges. 8. SURFACE
CONTAMINATION
Release of gaseous fission products to the extent of 10-l or 10V2of their rate of generation occurs through coatings which have been fractured, or completely penetrated by the diffusion of fuel. In the case of coatings of high integrity, release values of the order of 10-s or 1O-6 have been observed and have been correlated with fuel contamination in the coating. Surface contamination is defined as being within the range of cc-particle emission, i.e. ~15-20 ,u in pyrolytic carbon. The main causes of surface contamination are the formation of fuel dust by abrasion, thermal shock or reaction with impurities in the carrier and coating gases, with the subsequent incorporation of the dust within the coating, and the diffusion of fuel through the coating. It is therefore essential to adopt all possible measures to reduce these effects by for example maintaining the purity of the feed gases, ensuring that the fuel kernels are of adequate quality as regards crushing strength and abrasion resistance, charging the kernels into the coating furnace at a temperature such as to avoid excessive thermal shock, and applying the initial coating layers rapidly so as to protect the fuel kernels from abrasion effects. The diffusion of fuel can be minimized by coating at temperatures below those at which diffusion becomes significant, or alternatively by coating at high temperatures or methane concentrations so that the rate of deposition exceeds the rate ‘of diffusion.
FIG. 23.~Sintered zirconium-uranium monocarbide particle with interrupted laminar pyrolytic carbon, silicon carbide and interrupted laminar pyrolytic carbon coatings deposited at 1500°C. [X 1251
FIG.
24.-Porosity
or inclusions in layers of silicon carbide deposited with insufficient partial pressure of hydrogen. [ x 1251 638
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
639
8.1 Thorium-uranium dicarbidefuel Early results for the surface contamination (as determined by a-particle counting) of pyrolytic carbon coated kernels of thorium-uranium dicarbide showed such 6000 Kernel material: Natural thorium-uranium dicarbide x Laminar structure o Transitional structure v Columnar structure @Coating thickness (~1
5ooo
;@ I I I
I
4000.-
I
m :: 2 3000.. v) E z 0 A
I
zooo--
I
iooo-
0
FIG.
1400 1600 moo Temperature of deposition,
2S.-Dependence
2000 V
of a-count on temperature of deposition.
wide scatter that it was impossible to determine the effect of temperature
and rate of deposition of the pyrocarbon, or of the structure of the coating; or of kernel parameters such as alloying ratio, enrichment and physical form. Furthermore, the surface contamination was frequently affected by the formation of products of hydrolysis of thorium-contain@ carbide fuel, proving that the pyrolytic carbon was porous. Distinctive porosity has been observed at the junctions between adjacent cones of the columnar structure. A full understanding of the mechanism of the effect is not yet available but fuel coated with the more recently developed pyrocarbons or with an interlayer of silicon carbide is not susceptible to hydrolysis. The basic reasons for the relatively high a-activity of pyrolytic carbon coated thorium-uranium dicarbide fuel have been postulated by POINTUD(1964, private communication) from examination of the evidence available. He has pointed out that the thorium metal powder originally used in the preparation of fuel particles was characterized by the presence of highly active daughter products, e.g. 2MRa, in proportions depending on their half-lives and the period of storage since initial 2
640
J. R. C. Gouo~ and D. ,KERN
production of the powder. The radium can be expected to be volatilized at the high temperatures involved in the calcium reduction of thoria, and again in any subsequent high temperature heat treatment prior to the coating operation. Any radium present at the time of coating can be expected to diffuse rapidly through the pyrolytic carbon in a manner similar to barium or strontium. Therefore the a-activity of pyrolytic carbon coated thorium-uranium dicarbide fuel particles depends primarily on the time interval between completion of sintering of the kernels and completion of the coating operation, and on the relative rates of diffusion and carbon deposition. Experimental results from the coating of thorium-uranium dicarbide fuel with pyrolytic carbon have supplied supporting evidence. For example, the a-activity of a 1600°C laminar coating of thickness 80 p increased by a factor of 9 as the time interval from the end of heat treatment to the start of coating was increased from 90 min to one week. Measurements at various intervals of time after coating showed that the radioactive decay was characteristic of the daughter products of thorium. 8.1.1 E@ct of variations in the values of the coatingparameters. A study was made of the effects of variation in coating conditions on the extent of surface contamination of coated thorium-uranium dicarbide fuel. All fuel kernel batches were heat treated at 2200°C in vacua for 2 hr prior to coating in order to volatilize the highly a-active daughter of thorium. Figure 25 shows that for a constant methane concentration, the surface contamination increased linearly with temperature of deposition. For deposition temperatures above 1450°C the least contaminated coating was the metallographically laminar or structureless type deposited at a rate of >50 p/hr using a methane concentration of 30 per cent. For lower deposition rates ((20 p/hr for transitional and
Studies on the coating of fuel particles for the
DRAGON
reactor experiment
641
of halogens or halides to form and remove volatile compounds of uranium and thorium. Carbon tetrachloride vapour has the advantage that pyrolytic carbon is deposited concurrently in such a manner as to seal the surface of the coating. 15 ,u of pyrolytic carbon deposited from carbon tetrachloride vapour reduced the contamination of the laminar and columnar metallographic structures by about one order of magnitude ; a low level of contamination was obtained even with fuel kernels containing enriched uranium; and the improvement was comparable with the effect of an interlayer of silicon carbide. Pyrocarbon deposited from carbon tetrachloride vapour has several distinct advantages over layers of silicon carbide, which include a lower neutron absorption cross section, elimination of the hazards associated with the use of hydrogen, and simplification of chemical recycling processes. 9. CONCLUSIONS
The metallographic structure of pyrolytic carbon coatings on fuel kernels is sensitive to the conditions of fluidization, the concentration of hydrocarbon and the temperature of deposition. Temperature gradients in a laboratory-scale fluidized bed of fuel particles increase with the density of the particles and to a lesser extent with gas flow, but decrease with increased weight of charge. A modified graphite fluidization nozzle considerably reduced the temperature gradients. The carbon efficiency of the deposition process increases as deposition proceeds, probably as a result of the greater available surface area and changing conditions of fluidization. At constant coating temperature the carbon efficiency varies linearly with methane concentration, increasing in the zone of laminar deposition but decreasing in the zone of columnar deposition. This leads to a situation in which at a given temperature higher carbon efficiencies can be obtained with decreasing methane concentration, provided that the change in methane concentration is such as to change the metallographic structure of the deposited pyrocarbon. The rate of carbon deposition increases with the density of fuel kernels being coated, for constant weight of charge and within the range of coating parameters investigated. The rate of deposition at a given temperature was found to increase linearly with methane concentration, within each of two separate ranges of concentration corresponding to the different metallographic structures. An increase with temperature was observed for all methane concentrations, although previous data for the higher methane concentrations has shown that the rate of deposition reaches a maximum value and thereafter decreases with further temperature increase. The temperature giving the maximum rate of deposition was lower for the higher methane concentrations. The room temperature density values of vapour deposited pyrolytic carbons determined in the present studies show the characteristic minimum when plotted against temperature of deposition, although the temperatures of deposition 16001700°C giving the minimum density values are higher than those reported elsewhere. It is not the intention in this paper to discuss the basic reasons for the density minimum but it should be mentioned that the several contributory factors which have been suggested include the deviation of the apparent c-spacing from the ideal value, porosity formed by anisotropic thermal contraction from the deposition temperature, and very fine porosity detectable by electron microscopy. In addition, pore space
642
J. R. C. GOUGHand D. KERN
could be occupied by incompletely pyrolysed hydrocarbons. Other hypotheses to account for the density minimum are based on the mechanism of deposition of the pyrolytic carbon and the surface mobility of the deposited atoms or radicals at various temperatures. Interlayers of dense j3-silicon carbide, which are known to have favourable retention characteristics for certain metallic fission products are deposited from methyltrichlorosilane vapour in hydrogen carrier at a typical temperature of 1500°C. At higher temperatures or with insufficient partial pressure of hydrogen the apparent quality of the deposited silicon carbide deteriorates by the co-deposition of a-silicon carbide and of carbon. Coatings on fuel particles are assessed as regards metallographic structure, density, crushing strength, contamination in the surface layers as measured by acounting, and finally fission product release. The original metallographic classification of pyrolytic carbon as ‘laminar’, ‘structureless’ or ‘columnar’ is however being increasingly replaced by a more basic, structural classification in which the isotropy and crystallite size are determined as well as the density. The surface contamination of pyrolytic carbon-coated fuel particles depends, inter alia, on the rates of deposition of the pyrocarbon and diffusion of the fuel at the temperature of coating. Initially results showed an increase of surface contamination with temperature of coating within the range 1400-19OO”C,the increase with temperature being most rapid for the low rates of deposition giving the columnar structure. Recently however it has been shown that the very high rates of deposition possible using high methane concentrations at temperatures above 2000°C can result in surface contamination of a very low order. The incorporation of silicon carbide interlayers, and leaching with carbon tetrachloride vapour (which also results in the deposition of pyrolytic carbon) have been shown to give surface contamination often close to background level. Acknowledgments--The authors’ thanks are recorded to Mr. R. A. U. HUDDLE,Head of Materials Branch of the DRAGON Project,for his continual interest and guidance, to Mr. R. BICKERDIKE, Mr. H. BE~TLERand Mr. C. VIVANTE for useful discussions, and to Mr. C. A. RENNIE,Chief Executive of the DRAGON Project,for permission to publish this paper. REFERENCES B~KROSJ. C. (1964) The Structure of Pyrolytic Carbon Deposited in a FluidisedBed, Report GA-5163. Bormos J. C. and PRICER. J. (1965) Radiation-Induced Dimensional Changes in PyroIytic Carbons Deposited in a Fluidised Bed, Report GA-6736 CARL.EY-MACAULEY K. W. and MACKENZIE M. (1963) Studies on the Deposition of Pyrolytic Carbon, Proceedings of the Fzyth Carbon Conference, Vol. 2, p. 449, Pergamon Press, Oxford. DIEP~NWRPR. J. (196O)J. chim. Phys. 57,815. R. H., PAT~SONJ., P IJMMERY F. C. W. and W~rrs R. E. (1962) Some Measurements of FLOW the Diffusion of =U Fission Products and =Pa from ‘Coated ‘Fuels, Report AERE-R 4215. NORDWALL H. J. DE(1967) Private communication. POINTUDR. (1964). Private co~unicatiOn. Prwx M. S. T., G~UGH J. R C. and HORSLEY G. W. (1966)J. Br. Nucl. Energy Sot. 5,360. Proceedings of the Symposium on Ceramic Matrix Fuels Containing Coated Particles (1962) Battelle Memorial Institute, Columbus, Ohio, U.S.A., November 1962, TID/7654. SAYERS J. B., ROSEK. S. B., C!ooBSJ. H., HAUSER G. P. and VIVANTE C. (1963) Carbides in Nuclear Energy, Vol. 2, p. 919, Macmillan, London.